The invention relates to an automatic machine tool equipped with coupling devices which allow automatic replacement of end effectors so that a number of different tasks such as those involved in the assembly of aircraft panels can be performed in succession, the entire cycle of necessary jobs such as drilling, countersinking, glueing, rivet driving and hammering or pressing being performed in rapid succession, fully automatically, with no need to move the panel or the principal axes of the machine or both.
For the sake of clarity, the meaning of the technical terms used herein is set out below.
END EFFECTOR (EE): single electrically/pneumatically/hydraulically operated mechanical unit (effector) designed to perform preset basic operations
MULTITASK END EFFECTOR (MTEE): multiple interchangeable mechanical unit (effector) comprising one or more electrically/pneumatically/hydraulically operated EE designed to perform preset operation sequences
AIRCRAFT PANEL: fuselage and/or wing surface structure subassembly
TWIST HEAD: bi-rotating operating head
PIVOT POINT (PP): central pivoting point of the twist head
TOOLING: all fixtures, rests and end effectors
CNC: computerised numerical control
FASTENERS: elements used to effect rigid assembly of different parts of mechanical sub-assemblies, comprising pop rivets, solid rivets, bolts, hi-lock rivets, lock-bolt rivets, LGP rivets, anchor nuts and temporary fastenings such as log bolts, blind rivets, etc.
During the assembly of aircraft structures, and in particular during panel assembly, considerable difficulties are encountered because various operations need to be performed to extremely fine tolerances on pieces which are difficult to position and maneuver such as panels.
While the various components of the aircraft structure, and in particular the panels, constitute a unit resistant to considerable stresses once assembly is completed, they are extremely delicate and need to be handled with great care before assembly. In view of the fact that the length and/or width of the panels is often between 5 and 12 meters while the thickness is only 2 or 3 millimeters, that they present a curved surface, and that the various assembly operations have to be performed to tolerances of a tenth of a millimeter, the difficulties encountered are evident. For example, in order to assemble the metal sheets with the ribs and spars constituting the framework of an aircraft, or join the panels thus obtained one to another, fasteners such as nails or rivets are used. In order to apply them a hole must be drilled, then countersinking is performed and operations on the hole surface such as pretensioning are carried out; where necessary a layer of adhesive is applied, and finally the fastener is installed. All these operations must necessarily be performed with the greatest precision and speed.
For example, the tolerances required when driving some types of rivet must not exceed one or two hundredths of a millimeter as the rivet would otherwise be forced against the hole surface, damaging it and causing the panel to be scrapped. Once hammered in the rivet must be flush with the outer edge of the panel, neither projecting nor receding, in order to comply with aerodynamic requirements.
In order to perform all these operations with the necessary precision manual technology is used, supported by highly complex, costly equipment including automatic dedicated machine tools fitted to specially made units. These machines can be divided into two major categories--in the first case, the panel is fixed to mobile support structures to allow the machine to reach the fastener installation positions, while in the second, the machine moves and the panel does not. Both these systems present considerable limitations and drawbacks, however.
In the case of machines falling into the first category, the panels machined are secured to huge mobile supports. Great difficulty is obviously encountered in making structures to which curved panels only a few millimeters thick with an area of 50-60 sq.m. can be secured; the panels must be positioned on the structure with great precision and secured to it in such as way as to prevent even the slightest relative movement during machining. The result is a very large, heavy, complex structure which is required to move to very fine positioning tolerances in order to bring the various parts of the panel to the tool as required. This system also presents the serious drawback of requiring very long machine stoppages whenever the tooling has to be replaced so that a different job can be performed, e.g. in order to change over from drilling to rivet driving.
For this purpose it is necessary to shut down the entire machine, change the tool holders, tools and fixtures manually, and then clear the machine, i.e. reset all references, mechanical and electronic zeros etc. before starting on a different job. For this reason it is necessary to perform a given job on one panel, then remove it from the support and store it and proceed with the same job on the next panel. When all panels are finished the machine is shut down, the necessary tools replaced and the panels repositioned one at a time on the support ready for the next task to be performed.
The limitations and drawbacks of the dedicated machines described above can be summarised as follows:
(a) very large mobile structures have to be constructed to perform controlled movements with fine tolerances
(b) units have to be shut down for long periods whenever a different job is performed and the tooling has to be changed
(c) the unit lacks flexibility because it is designed to do a given job and cannot be adapted for different operations
(d) it takes a long time to finish each piece
(e) a great deal of space is required to store a given number of panels between one operation and the next
(f) the use of the machine is limited in flexibility as the most complex operations cannot be performed, especially in the case of sharply curving parts, partly because of interference between the workpiece, its rest and the machine.
The machines in the second category, one of which is described in Italian patent application No. 44804 a/88, comprise a mobile arm whose length is equal to three perpendicular axes, at the end of which is fitted a twist head with one or two degrees of freedom; this head can rotate around polar axes which thus constitute the last axes of the unit.
The end effector is fitted on the twist head; the effector may, for example, be represented by an electric chuck with corresponding automatically interchangeable tool. While this type of apparatus can be adapted to perform different jobs more easily than the first type, it does not wholly solve the problems described above. Although the panel remains fixed during machining, replacement of the end effectors is highly laborious and their structure is complex, as they must be equipped with various effectors for the positioning sequences of the individual units. In addition, the constraints due to the geometry of the head cause problems with automatic fastener feed connection, and require excessive lengths at the expense of precise positioning. This obviously means greater difficulty in repositioning the tool on the workpiece while remaining within the required tolerances.
There is consequently a strongly-felt need for a machine which automatically performs a succession of different tasks and features simplified replacement of the end effectors without shutting down the machine or moving the workpiece.